Method for converting hydrocarbon feedstocks by means of thermal steam cracking

Abstract

The invention relates to a process for converting hydrocarbon feeds by thermal steamcracking to at least one olefin-containing product stream comprising at least ethylene and propylene, with at least partial conversion of a first hydrocarbon feed in at least one first cracking furnace ( 1 ) and of a second hydrocarbon feed in at least one second cracking furnace ( 2 ). According to the invention, the second hydrocarbon feed comprises predominantly hydrocarbons having a carbon number of 5 or/and 4 and consists for the most part of one or more recycled fractions (P, T) which are obtained from the product stream, and the second hydrocarbon is converted in the second cracking furnace ( 2 ) with cracking conditions that lead to a ratio of propylene to ethylene of 0.7 to 1.6 kg/kg.

The invention claimed is: 
     
       1. A process for converting hydrocarbon feeds by thermal steamcracking to an olefin-containing product stream comprising at least ethylene and propylene, with at least partial conversion of a first hydrocarbon feed in at least one first cracking furnace ( 1 ) and of a second hydrocarbon feed in at least one second cracking furnace ( 2 ), characterized in that the second hydrocarbon feed comprises predominantly hydrocarbons having a carbon number of 5 or/and 4 and consists for the most part of one or more recycled fractions (P, T) which are obtained from the product stream, the second hydrocarbon being converted in the second cracking furnace ( 2 ) with cracking conditions that lead to a ratio of propylene to ethylene of 0.85 to 1.6 kg/kg, the first hydrocarbon feed being supplied with at least one fraction (U) which has been separated from the product stream and is recycled, comprising predominantly hydrocarbons having a carbon number of at least 6, the first hydrocarbon feed being converted in the first cracking furnace ( 1 ) with cracking conditions that lead to a ratio of propylene to ethylene of 0.25 to 0.85 at the cracking furnace outlet, and the value for the ratio of propylene to ethylene for the second hydrocarbon feed being above the value for the ratio of propylene to ethylene for the first hydrocarbon feed. 
     
     
       2. The process as claimed in  claim 1 , characterized in that the second hydrocarbon consists exclusively of one or more recycled fractions (P, T). 
     
     
       3. The process as claimed in  claim 1 , characterized in that the hydrocarbons having a carbon number of 5 present in the second hydrocarbon feed are predominantly saturated hydrocarbons. 
     
     
       4. The process as claimed in  claim 1 , characterized in that the second hydrocarbon is substantially free of diolefins. 
     
     
       5. The process as claimed in  claim 1 , characterized in that the second hydrocarbon feed comprises predominantly saturated hydrocarbons. 
     
     
       6. The process as claimed in  claim 1 , characterized in that the second hydrocarbon is converted in the second cracking furnace ( 2 ) with cracking conditions that lead to a ratio of propylene to ethylene of up to 1.2 kg/kg, at the cracking furnace outlet. 
     
     
       7. The process as claimed in  claim 1 , characterized in that the first hydrocarbon feed is converted in the first cracking furnace ( 1 ) with cracking conditions that lead to a ratio of propylene to ethylene of 0.3 to 0.75 kg/kg, preferably of 0.4 to 0.65 kg/kg, at the cracking furnace outlet. 
     
     
       8. The process as claimed in  claim 1 , in which the values for the ratio of propylene to ethylene for the first and second hydrocarbons differ by at least 0.1 kg/kg, preferably by at least 0.15 kg/kg, more preferably by at least 0.2 kg/kg. 
     
     
       9. The process as claimed in  claim 1 , in which the cracking furnace exit temperature for the conversion in the second cracking furnace ( 2 ) is between 680° C. and 820° C., preferably between 700° C. and 800° C. and further preferably between 710° C. and 780° C. and more preferably between 720° C. and 760° C., and the cracking furnace exit temperature for the conversion in the first cracking furnace ( 1 ) is between 800° C. and 1000° C., preferably between 820° C. and 950° C. and more preferably between 840° C. and 900° C., the cracking furnace exit temperature of the first cracking furnace ( 1 ) being above that of the second cracking furnace ( 2 ). 
     
     
       10. The process as claimed in  claim 9 , in which the cracking furnace exit temperature for the conversion in the first cracking furnace ( 1 ) is at least 10° C. above, preferably at least 15° C. above, more preferably at least 20° C. above, the cracking furnace exit temperature for the conversion in the second cracking furnace ( 2 ). 
     
     
       11. The process as claimed in  claim 1 , in which 0.3 to 1.5 kg of steam per kg of hydrocarbon feed is used in the first cracking furnace ( 1 ), and 0.15 to 0.8 kg of steam per kg of hydrocarbon feed in the second cracking furnace ( 2 ). 
     
     
       12. The process as claimed in  claim 1 , in which at least one fraction (V) comprising predominantly hydrocarbons having a carbon number of 2 or 3 is obtained from the product stream and at least partly converted in a cracking furnace ( 3 ) for gaseous feed. 
     
     
       13. The process as claimed in  claim 1 , characterized in that the fresh feed (B) used for the first hydrocarbon feed comprises natural gas condensates and/or crude oil fractions, especially naphtha, and/or synthetic and/or biogenic hydrocarbons and/or mixtures derived therefrom.